Single column ion chromatography, termed “SCIC” herein, encompasses ion chromatography applications that do not use a suppressor. In the early days of ion chromatography a packed bed suppressor led to several potential problems such as added delay volume causing band broadening, a dependence of the signal on the degree of exhaustion of the packed bed, a need to frequently regenerate the bed, and the like. Therefore, some investigators investigated methods of ion chromatography without the suppressor. See U.S. Pat. Nos. 4,272,246 and 4,732,686.
Eluent or mobile phase recycle in HPLC is predominantly pursued in the isocratic mode by collecting fractions during the analysis and targeting regions in the chromatogram where there are no peaks. Typically a valve is switched to divert the analyte peaks to waste while collecting the mobile phase during the baseline portion of the chromatogram and diverting it back to the mobile phase reservoir. While this approach is commonly used, it suffers from several limitations such as (a) the approach is only feasible when the analyte levels are significantly higher than the baseline noise, (b) since the method relies on distinguishing the baseline portion from the peaks of interest and when the analyte levels are close to the minimum detection limits the differences become small and it is difficult to cleanly fractionate the solvent portion, and (c) the method works well when the number of peaks in the chromatogram are low since it relies on the availability of regions that do not contain the analytes. There are several commercial modules that are available for the solvent recycling such as a module called S3-Solvent Recycler from Spectrum Corporation, Houston, Tex., USA.
Early workers also directly recycled the mobile phase after separation by diverting the waste liquid back into the mobile phase reservoir. Since the analytes are also recycled in this approach along with eluents only limited amount of recycle was possible. Additionally when the analyte ions were present at significant concentrations they affected the separation and the background.
An ion chromatography approach to recycle was investigated by Yokoyama et. al (J. Chromagr. A 1089, (2005), 82-86. In this approach an aliquot of mixed bed resin was added to the eluent reservoir and stirred. When a packed column embodiment with the mixed bed resin was used in the above setup poor analyte removal was observed and the baseline drifted upwards. The analyte ions were eluted off the mixed bed ion exchange column by the eluent. Adding the mixed bed resin to the eluent reservoir however yielded improved performance as evident from being able to operate with 5× lower eluent volume. The approach however relied on equilibration of the resin with the analyte ions for removal, and a stirrer was needed to ensure good contact of the resin with the analyte ions dissolved in the eluent.
There is a need for an improved SCIC system.